A PDP utilizing a discharge emission phenomenon is being developed as a flat display which is easily increased in size. In an AC-type PDP having a structure comprising a transparent electrode covered with a glass dielectric, a protective film is generally formed on the dielectric to prevent the increase of discharge voltage due to damage of a surface of the dielectric by sputtering of ion bombardment. This protective film is required to have a low discharge voltage and an excellent resistance to sputtering.
A MgO film has conventionally been used as a protective film that satisfies the above requirement. The MgO film is an insulator having an excellent resistance to sputtering and a large emission coefficient of secondary electrons, and hence can lower discharge start voltage, thus extending the life of a PDP.
Currently, a MgO film is generally formed by depositing a film on a dielectric using a MgO deposition material by a vacuum deposition method, such as an electron beam deposition method or an ion plating method. As a MgO deposition material, for example, a material obtained by pulverizing a sintered material of high-purity polycrystalline MgO or a MgO single crystal is used.
The sintered material of polycrystalline MgO has a low deposition rate, and is likely to splash during the deposition when it is used as a deposition material, which makes it difficult to obtain a uniform protective film. For solving the problem, sintered pellets of polycrystalline MgO having high purity and high density and having an average crystal grain size controlled in a specific range, or sintered pellets of polycrystalline MgO having high purity and high density and having a specific carbon content or less have been proposed wherein the sintered pellets are unlikely to splash during the vapor deposition and forming a uniform protective film, when they are used as a deposition material (patent documents 1 and 2).
Further, a deposition material comprised of a sintered material of polycrystalline MgO having a specific volume or surface roughness has been proposed in order to increase a substantial surface area of the deposition material of a region irradiated with an electron beam for improving the deposition rate (patent documents 3, 4 and 5). A sintered material of polycrystalline MgO having a deposition rate improved by dispersing a specific amount of an alkaline earth metal oxide in the material has been proposed (patent document 6).
The sintered materials of polycrystalline MgO obtained by the above improved method improved deposition rates during the vapor deposition or prevented from splashing to some extent, but a satisfactory protective film cannot be obtained. In addition, the polycrystalline sintered material inherently has lattice strain concentrated in a grain boundary, and is likely to have an uneven grain boundary concentration exposed through the surface of the deposition material, which causes a fundamental problem in that amounts of evaporation of the MgO varies easily.
On the other hand, as a method for obtaining a MgO single crystal deposition material with high productivity, a method in which a MgO single crystal is pulverized by impact force of a rotating cutter is employed. The deposition material obtained by pulverizing a MgO single crystal has a relatively high deposition rate, and forms an excellent protective film. However, the MgO single crystal deposition material produced by the pulverizing method often splashes during the deposition due to its indefinite form. Therefore, particularly when deposited on a large-size substrate, a problem occurs in that it is difficult to obtain a protective film having uniform quality.
For solving the problem, a particle size of MgO is optimized depending on a deposition machine and deposition conditions to obtain a good balance between the deposition rate and the frequency of splashing, improving both the productivity and the quality of film. The MgO single crystal having a particle size optimized is improved in deposition rate; however, it cannot be satisfactorily prevented from splashing during the deposition, and hence is not satisfactory from the viewpoint of achieving uniformity of the MgO film ultimately obtained.
A MgO deposition material having a water resistance improved by controlling the calcium oxide (CaO) content and silicon dioxide (SiO2) content of a MgO single crystal and a ratio between them has been proposed (patent document 7). When this deposition material is used, the time to achieve a certain vacuum range is shortened, but it is difficult to obtain a uniform protective film having excellent quality.
The MgO single crystal is not only used as a deposition material but also frequently used as a substrate on which an oxide superconductor thin film is formed since the MgO single crystal has excellent lattice match with an oxide superconductor and further has a thermal expansion coefficient that is consistent with that of an oxide superconductor. Particularly, the MgO single crystal has a low permittivity and a small dielectric loss at a high frequency, and therefore has attracted attention as a substrate of an oxide superconductor thin film for use in a high frequency device.
A variety of methods for improving the above MgO single crystal substrates for oxide superconductors, for example, a method of obtaining a single crystal having a large size (patent documents 8 and 9), a method of obtaining a single crystal having excellent crystallinity (patent document 10), a method of improving surface properties of a substrate (patent documents 11, 12, and 13), and others have been proposed. However, a satisfactory oxide superconductor thin film cannot be obtained.
[Patent document 1] Japanese Unexamined Patent Publication No. Hei 10-297956
[Patent document 2] Japanese Unexamined Patent Publication No. 2000-63171
[Patent document 3] Japanese Unexamined Patent Publication No. 2004-43955
[Patent document 4] Japanese Unexamined Patent Publication No. 2004-43956
[Patent document 5] Japanese Unexamined Patent Publication No. 2004-84016
[Patent document 6] Japanese Unexamined Patent Publication No. 2000-290062
[Patent document 7] Japanese Unexamined Patent Publication No. 2000-103614
[Patent document 8] Japanese Unexamined Patent Publication No. Hei 02-263794
[Patent document 9] Japanese Unexamined Patent Publication No. Hei 05-170430
[Patent document 10] Japanese Unexamined Patent Publication No. Hei 06-405887
[Patent document 11] Japanese Unexamined Patent Publication No. Hei 09-309799
[Patent document 12] Japanese Unexamined Patent Publication No. 2000-86400
[Patent document 13] Japanese Unexamined Patent Publication No. Hei 11-349399